In a scroll compressor the trapped volumes are in the shape of lunettes and are defined between the wraps or elements of the fixed and orbiting scrolls and their end plates. The lunettes extend for approximately 360.degree. with the ends of the lunettes defining points of tangency or contact between the wraps of the fixed and orbiting scrolls. These points of tangency or contact are transient in that they are continuously moving towards the center of the wraps as the trapped volumes continue to reduce in size until they are exposed to the outlet port. As the trapped volumes are reduced in volume the ever increasing pressure acts on the wrap and end plate of the orbiting scroll tending to axially and radially move the orbiting scroll with respect to the fixed scroll.
Radial movement of the orbiting scroll away from the fixed scroll is controlled through radial compliance. Eccentric bushings, swing link connections and slider blocks have all been disclosed for achieving radial compliance. Each approach ultimately relies upon the centrifugal force produced through the rotation of the crankshaft to keep the wraps in sealing contact.
Axial movement of the orbiting scroll away from the fixed scroll produces a thrust force. The weight of the orbiting scroll, crankshaft and rotor may act with, oppose or have no significant impact upon the thrust force depending upon whether the compressor is vertical or horizontal and, if vertical, whether the motor is above or below the orbiting scroll. Also, the highest pressures correspond to the smallest volumes so that the greatest thrust loadings are produced in the central portion of the orbiting scroll but over a limited area. The thrust forces push the orbiting scroll against the crankcase with a large potential frictional loading and resultant wear. A number of approaches have been used to counter the thrust forces such as thrust bearings and a fluid pressure back bias on the orbiting scroll. Discharge pressure and intermediate pressure from the trapped volumes as well as an external pressure source have been used to provide the back bias. Specifically, U.S. Pat. Nos. 3,600,114, 3,924,977 and 3,994,633 utilize a single fluid pressure chamber to provide a scroll biasing force. This approach provides a biasing force on the orbiting scroll at the expense of very large net thrust forces at some operating conditions. As noted, above, the high pressure is concentrated at the center of the orbiting scroll but over a relatively small area. If the area of back bias is similarly located, there is a potential for tipping since some thrust force will be located radially outward of the back bias. Also, with the large area available on the back of the orbiting scroll, it is possible to provide a back bias well in excess of the thrust forces.